1. Technical Field
This invention relates to an apparatus for sensing whether a liquid with a turbulent surface and contained within a vessel has fallen to a level in the vessel where the liquid is substantially expended, and, in one specific embodiment to, sensing whether the ink in the pen of a pen plotter is substantially expended while the pen is in motion.
2. Background Art
Pen plotters are well known in the plotting art. As depicted in FIG. 1, a typical pen plotter includes a pen carriage 10 which slides laterally on a support beam 12 to produce vectors in one axis of the plotter's coordinate system. The pen carriage 10 carries a pen 14 in its gripping fingers 16. The pen 14 is lifted and lowered by a mechanism 15. The pen 14 is filled with a liquid ink which flows from the tip 18 onto the plotting media 20 to create the plot. Over time, therefore, the ink within the pen 14 is consumed. To assure that the pen 14 does not run out of ink during a plot, therefore, the level of the ink remaining within the pen 14 should be checked periodically.
In FIG. 1, the pen 14 and pen carriage 10 are positioned over the plot portion of the plotting media 20. As depicted in FIG. 2, in prior art plotters the pen 14 and pen carriage 10 are moved from the plotting position 22 to an off-line position 24 for ink level sensing. A level sensing system 26 is located at the off-line position 24. The level sensing system 26 can be optical or capacitance in operation, by way of example. In an optical system, a light source 28 transmits a light beam 30 through the transparent body of a pen 14 to be received by a light sensor 32 which generates an electrical signal on line 34 when the light beam 30 is not blocked by the opaque ink. The light source 28 and light sensor 32 are at a fixed level with respect to the body of the of the pen 14 corresponding to the level at which the pen 14 is out of or nearly out of ink. The sensing system 26, therefore, indicates when the level of the ink within the pen 14 has fallen to the aforementioned fixed level.
While the above-described level sensing system 26 works for its intended purpose, it has certain drawbacks. When the pen 14 has been in use, the ink may be frothing and partially coating the sides of the inside of the pen body such that some time must be allowed at the off-line position 24 for the ink to settle before making the level reading. Therefore, the plotting process must be stopped periodically to check the ink level. This, of course, takes time away from the plotting process.
In addition, the level of the ink within the pen at which the system is keyed to indicate that the pen 14 needs to be replaced must be chosen carefully. The lower the preset "empty" threshold is set, the less ink is wasted by changing pens before the ink is totally consumed. However, the lower the threshold is set, the more often the level testing must take place to assure that the pen 14 will not run out of ink during plotting. Since the plotting must be stopped to allow the ink to settle, considerable amounts of plotting time would be lost if the ink level is checked frequently. Consequently, the total throughput of the plotter goes down. On the other hand, if the ink level is tested less frequently, the threshold must be set high enough to ensure that adequate amounts of ink remain between tests, so that the pen 14 will not run out of ink during plotting. Consequently, ink is wasted by replacing the pen 14 while significant amounts of ink still remain. Accordingly, there is a tradeoff between wasting ink and losing plotting time.
An optimum approach would be to sense the ink level of the pen 14 as it is gripped by the gripping fingers 16 and in motion on the pen carriage 10. Unfortunately, the above-described frothing and coating action of the ink within the body of the pen 14 has made such dynamic testing of the ink level impossible in the prior art.
Wherefore, it is an object of the present invention to provide a method and apparatus for sensing if the level of ink within a plotter pen has fallen to a level where the ink is substantially expended while on the pen carriage.
It is another object of the present invention to provide a method and apparatus for sensing if the level of ink within a plotter pen has fallen to a level where the ink is substantially expended, while the pen carriage is in use.
It is still another object of the present invention to provide a method and apparatus for sensing if the level of ink within a plotter pen has fallen to a level where the ink is substantially expended without the need to allow frothing and coating ink to settle within the pen body before measuring.
It is yet another object of this invention to provide a method and apparatus for sensing if the level of ink within a plotter pen has fallen to a level where the ink is substantially expended such that a minimum of ink is wasted without the lose of plotting time.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.